A simple sensitive method has been developed for the assay of prostacyclin. This involves the incubation of samples containing prostacyclin with platelet rich plasma (PRP) for 1 min at 37 degrees from human blood. The amount of cyclic AMP formed during the incubation with prostacyclin was measured by specific radioimmunassay. Prostacyclin induced a marked increase in cyclic AMP in a dose-dependent fashion, 50% of the maximal increase was produced with 50 pmol of prostacyclin/ml. This assay is sensitive enough to measure concentrations of prostacyclin as low as 10-20 pmols. Treatment of prostacyclin by acid completely destroys its ability to increase cyclic AMP. This test was used to show the specificity of the measurement of prostacyclin in various biological samples. This method has been used to measure the prostacyclin synthesis from arachidonic acid in various tissues such as lung, seminal vesicles and stomach fundus.